Abstract
We have systematically investigated the dependence of the crystal structure and the microwave response of laser ablated YBa2Cu3Oz (YBCO) thin films on MgO substrate microstructure formed upon thermal annealing. A dramatic improvement in both crystallinity as well as microwave surface resistance Rs (of almost an order of magnitude) is observed for the films grown on substrates annealed for the initial 2 h at 1000 °C in flowing O2 ambient compared to films deposited on non-annealed substrates. However, for further annealing of the MgO substrate we found no further pronounced improvement in either Rs or the crystalline quality of YBCO. The observed improvement in Rs for the film grown on an annealed substrate can be explained by the graphoepitaxial growth of YBCO on MgO that results in very good crystallinity of YBCO film. The ratio of 0° and 45° grains, D(45°) and the degree of in-plane alignment, Δϕ, have a non-monotonic relation with the substrate annealing time. At the same time, a strong correlation is observed between Rs and Δϕ, and D(45°), which themselves are dependent on annealing time.
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